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We study, within the fluctuation exchange approximation, thespin-fluctuation-mediated superconductivity in Hubbard-type models possessingelectron and hole bands, and compare them with a model on a square lattice witha large Fermi surface. In the square lattice model, superconductivity is moreenhanced for better nesting for a fixed band filling. By contrast, in themodels with electron and hole bands, superconductivity is optimized when theFermi surface nesting is degraded to some extent, where finite energy spinfluctuation around the nesting vector develops. The difference lies in therobustness of the nesting vector, namely, in models with electron and holebands, the wave vector at which the spin susceptibility is maximized is fixedeven when the nesting is degraded, whereas when the Fermi surface is large, thenesting vector varies with the deformation of the Fermi surface. We alsodiscuss the possibility of realizing in actual materials the bilayer Hubbardmodel, which is a simple model with electron and hole bands, and is expected tohave a very high T_c.

Finite energy spin fluctuation as a pairing glue in systems with coexisting electron and hole bands